Why would Bashar al-Assad use chemical weapons in defiance of clear threats by President Obama to intervene if he did so?
Dictators are often prone to taking risks: this is because they are generally invulnerable at home because of their security measures and repression. They are also surrounded by yes men who give them bad advice by anticipating what they think the dictator wants to hear and then saying it. Because of the dictator's paranoid tendencies in some cases, this is a natural survival strategy on the part of his advisers.
